1. Field of the Invention
The present invention relates to a power supply, and more particularly to a power supply having a surge protection circuit.
2. Description of the Related Art
Despite strict rules for ensuring safety and protection regarding power transmission and power supply in a modern society, there is no guarantee that power-receiving devices can be totally immune from damage throughout the course of power supply. The reason mostly lies in the intervention of natural power, such as lightning.
With reference to FIG. 4, a conventional power supply 90 has an AC power input terminal and a power output terminal connected with a ground terminal. The conventional power supply 90 serves to convert the mains power into an operating power for a load 992. The power supply 90 has a transformer 96, a voltage dependent resistor (VDR) 91, a common mode (CM) inductor 92, a rectification circuit 93, an input capacitor 94, a switch 95, an output capacitor 97 and a Y capacitor 98. The VDR 91, the common mode inductor 92, the rectification circuit 93, the input capacitor 94 and the switch 95 are parallelly connected with a primary side of the transformer 96. The output capacitor 97 is connected with the secondary side of the transformer 96, and both sides of the output capacitor 97 are connected to the power output terminal of the power supply 90 and the load 992. The Y capacitor 98 is connected between the primary side and the secondary side of the transformer 96 to cancel common mode electromagnetic interference (EMI). The AC power input terminal has a live line (L) and a neutral line (N). The VDR 91 is connected between the live line (L) and the neutral line (N) for the purpose of surge protection.
With reference to FIG. 5, the circuit is operated as follows. After the mains power enters from the AC power input terminal, the CM inductor 92 filters the CM noise of the mains power and the rectification circuit 93 converts the mains power into DC power. The switch 95 is connected to an output terminal of the rectification circuit 93. The DC power passing through the switch 95 is outputted to the primary side of the transformer 96 so that the secondary side of the transformer 96 can generate an induced voltage to be outputted to the power output terminal.
When lightning occurs, the energy of lightning generates a tremendously high voltage difference within a short period of time. According to the impedance characteristics of capacitors, a capacitor is considered in a state of short-circuit when subjected to such an abrupt and drastic voltage variation. Hence, when a surge voltage arising from lightning occurs between the primary side and the secondary side of the transformer 96, the Y capacitor is in a conducting state so that a surge current generated by the high voltage difference sequentially flows from the live line (L) to the ground terminal through the CM inductor 92, the rectification circuit 93, the input capacitor 94 and the Y capacitor 98. As a consequence, the CM inductor 92, the rectification circuit 93 and the input capacitor 94 are damaged by the surge current flowing through, and this causes the power supply 90 to fail.